BepiColombo, the ESA/JAXA joint mission performed its first flyby of Mercury on 1 October 2021 and its second on 23 June 2022. PHEBUS observed the exosphere of Mercury during these flybys notably ...with its visible channels c404 (centered on the potassium emission line at 404 nm) and c422 (centered on the calcium emission line at 422 nm). The c422 signal shows not only an enhancement of calcium (Ca) near the dawn region but also a very extensive Ca exosphere on the morning side beyond 10,000 km. The e‐folding distance deduced from our Ca profiles (2,500–2,800 km) is in agreement with the value reported by MESSENGER at similar true anomaly angles. We use a Chamberlain model to determine the temperature and density at the exobase. Using the morning side low‐altitude data, we derived a high temperature at the exobase (>50,000 K), in agreement with the MESSENGER results. We also report a day/night asymmetry in the Ca exosphere that could indicate that the source of Ca is predominantly on the dayside or be the consequence of a shift of the main source of Ca away from the dawn region. The c404 channel detected additional species at low altitudes on the morning side during both flybys. Comparison with previous studies is inconclusive and further analysis will be necessary to identify the species. Nevertheless, we can note that the e‐folding distance deduced from our profile is relatively small (135 km) and that the Chamberlain model applied to our profiles seems to indicate a temperature at the exobase <3,000 K.
Plain Language Summary
BepiColombo, the ESA/JAXA joint mission on its way to Mercury, has already flown over the planet twice in October 2021 and June 2022. PHEBUS (Probing of Hermean Exosphere By Ultraviolet Spectroscopy), the UV spectrometer onboard the spacecraft, observed Mercury's exosphere during the flybys at the closest parts of approach to the planet with its two visible channels notably. Mercury is surrounded by a tenuous collisionless atmosphere (exosphere) that contains a variety of species. We report the detection of exospheric calcium with an enhancement near the dawn region. We show that the calcium exosphere is much extended on the morning side, beyond 10,000 km. The observations also reveal a day/night asymmetry of the calcium exosphere. Furthermore, we report the detection of additional species. However, the flyby data alone do not allow us to formally identify the species.
Key Points
PHEBUS observed Mercury's extended exosphere of calcium on the morning side beyond ∼10,000 km
The observed calcium profiles may be explained by the 2D effects of altitude and local time
Additional species were detected, potentially manganese and potassium
Global water production rates were determined from the Ly Delta *a emission of hydrogen around comet 103P/Hartley 2, observed with the SWAN (Solar Wind Anisotropies) all-sky camera on the SOHO ...spacecraft from 2010 September 14 through December 12. This time period included the November 4 flyby by the EPOXI spacecraft. Water production was three times lower than during the 1997 apparition also measured by SWAN. In 2010, it increased by a factor of ~2.5 within one day on September 30 with a similar corresponding drop between November 24 and 30. The total surface area of sublimating water within ?20 days of perihelion was ~0.5 km2, about half of the mean cross section of the nucleus. Outside this period it was ~0.2 km2. The peak value was 90%, implying a significant water production by released nucleus icy fragments.
SWAN, the Solar Wind ANisotropies all-sky hydrogen Ly Delta *a camera on the Solar and Heliospheric Observatory spacecraft that makes all-sky images of interplanetary neutral hydrogen, has an ongoing ...campaign to make special observations of comets, both short- and long-period ones, in addition to the serendipitous observations of comets as part of the all-sky monitoring program. We report here on a study of several short-period comets that were detected by SWAN: 21P/Giacobini-Zinner (1998 and 2005 apparitions), 19P/Borrelly (2001 apparition), 81P/Wild 2 (1997 apparition), and 103P/Hartley 2 (1997 apparition). SWAN observes comets over long continuous stretches of their visible apparitions and therefore provides excellent temporal coverage of the water production. For some of the observations we are also able to analyze an entire sequence of images over many days to several weeks/months using our time-resolved model and extract daily average water production rates over continuous periods of several days to months. The short-term (outburst) and long-term behavior can be correlated with other observations. The overall long-term variation is examined in light of seasonal effects seen in the pre- to post-perihelion differences. For 21P/Giacobini-Zinner and 81P/Wild 2 the activity variations over each apparition were more continuously monitored but nonetheless consistent with previous observations. For 19P/Borrelly we found a very steep variation of water production rates, again consistent with some previous observations, and a variation over six months around perihelion that was reasonably consistent with the spin-axis model of Schleicher et al. and the illumination of the main active areas. During the 1997-1998 apparition of 103P/Hartley 2, the target comet of the EPOXI mission (the Deep Impact extended mission), we found a variation with heliocentric distance (~r --3.6) that was almost as steep as 19P/Borrelly and, given the small measured radius near aphelion, this places a number of possible constraints on the size, shape, and/or distribution active of areas on the surface.
In 2017, 2018, and 2019, comets 46P/Wirtanen, 45P/Honda-Mrkos-Pajdusakova, and 41P/Tuttle-Giacobini-Kresak all had perihelion passages. Their hydrogen comae were observed by the Solar Wind ...ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliospheric Observer (SOHO) satellite: comet 46P for the fourth time and comets 45P and 41P for the third time each since 1997. Comet 46P/Wirtanen is one of a small class of so-called hyperactive comets whose gas production rates belie their small size. This comet was the original target comet of the Rosetta mission. The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliospheric Observer (SOHO) satellite observed the hydrogen coma of comet 46P/Wirtanen during the apparitions of 1997, 2002, 2008, and 2018. Over the 22 years, the activity decreased and its variation with heliocentric distance has changed markedly in a way very similar to that of another hyperactive comet, 103P/Hartley 2. Comet 45P/Honda-Mrkos-Pajdusakova was observed by SWAN during its perihelion apparitions of 2001, 2011, and 2017. Over this time period the activity level has remained remarkably similar, with no long-term fading or abrupt decreases. Comet 41P/Tuttle-Giacobini-Kresak was observed by SWAN in its perihelion apparitions of 2001, 2006, and 2017 and has decreased in activity markedly over the same time period. In 1973 it was known for large outbursts, which continued during the 2001 (2 outbursts) and 2006 (1 outburst) apparitions. However, over the 2001 to 2017 time period covered by the SOHO/SWAN observations the water production rates have greatly decreased by factors of 10-30 over corresponding times during its orbit.
BepiColombo is a joint mission between the European Space Agency, ESA, and the Japanese Aerospace Exploration Agency, JAXA, to perform a comprehensive exploration of Mercury. Launched on
20
th
...October 2018 from the European spaceport in Kourou, French Guiana, the spacecraft is now en route to Mercury.
Two orbiters have been sent to Mercury and will be put into dedicated, polar orbits around the planet to study the planet and its environment. One orbiter, Mio, is provided by JAXA, and one orbiter, MPO, is provided by ESA. The scientific payload of both spacecraft will provide detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. Mercury is the planet closest to the Sun, the only terrestrial planet besides Earth with a self-sustained magnetic field, and the smallest planet in our Solar System. It is a key planet for understanding the evolutionary history of our Solar System and therefore also for the question of how the Earth and our Planetary System were formed.
The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere, and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein’s theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload such that individual measurements can be interrelated and complement each other.
We present a method to derive outflow velocities in the solar corona using different data sets, including solar wind mass flux coming from the SWAN SOHO instrument, electron density values from ...LASCO-C2, and interplanetary solar wind velocities derived from ground-based interplanetary scintillation observations (IPS). In a first step, we combine the LASCO electron densities at 6 R unk and the IPS velocities and compare the product to the SWAN mass fluxes. It is found that this product represents the actual mass flux at 6 R unk for the fast wind, but not for the slow wind. In regions dominated by the slow wind, the fluxes derived from SWAN are systematically smaller. This is interpreted as proof that the fast solar wind has reached its terminal velocity at similar to 6 R unk and expands with constant velocity beyond this distance. On the contrary, the slow solar wind has reached only half of its terminal value and is thus accelerated farther out. In a second step, we combine the LASCO-C2 density profiles and the SWAN flux data to derive velocity profiles in the corona between 2.5 and 6 R unk. Such profiles can be used to test models of the acceleration mechanism of the fast solar wind.
In 2021 and 2022 the hydrogen comae of three long period comets, C/2020 S3 (Erasmus), C/2021 A1 (Leonard) and C/2021 O3 (PanSTARRS) were observed with the Solar Wind ANisotropies (SWAN) all-sky ...hydrogen Lyman-alpha camera on the SOlar and Heliosphere Observer (SOHO) satellite. SWAN obtains nearly daily full-sky images of the hydrogen Lyman-α distribution of the interstellar hydrogen as it passes through the solar system yielding information about the solar wind and solar ultraviolet fluxes that eats away at it by ionization and charge exchange. The hydrogen comae of comets, when of sufficient brightness, are also observed. Water production rates have been calculated over time for each of these comets. Of particular interest are comet C/2021 O3 (PanSTARRS) which apparently disintegrated a few days before its perihelion at 0.28 au and C/2021 A1 (Leonard) which also disintegrated beginning about 20 days after its perihelion peak. The behavior of comet C/2020 S3 (Erasmus) was more typical without dramatic fading, but still was asymmetric about perihelion, with a more rapid turn on before perihelion and more extended activity well after perihelion.
•SOHO SWAN obtained images of H Lyman-α in 3 long-period comets from 2020 to 2022.•The water production rate was calculated for each comet image.•Comet C/2020 S3 (Erasmus) varied asymmetrically around perihelion.•Comets C/2021 A1 (Leonard) and C/2021 O3 (PanSTARRS) disintegrated at radii of 121–192 m.
The Solar Wind ANisotropies (SWAN) all-sky hydrogen Ly alpha camera on the SOlar and Heliospheric Observer (SOHO) satellite observed the hydrogen coma of comet C/2012 S1 (ISON) for most of the last ...month of its activity from 2013 October 24 to November 24, ending just 4 days before perihelion and its final disruption. The water production rate of the comet was determined from these observations. SOHO has been operating in a halo orbit around the Earth-Sun L1 Lagrange point since its launch in late 1995. Most water vapor produced by comets is ultimately photodissociated into two H atoms and one O atom producing a huge hydrogen coma that is routinely observed in the daily SWAN images in comets of sufficient brightness. Water production rates were calculated from 22 images over most of the last month of the pre-perihelion apparition. The water production rate increased very slowly on average from October 24.9 until November 12.9, staying between 1.8 and 3.4 x 10 super(28) s super(-1), after which it increased dramatically, reaching 1.6 to 2 x 10 super(30) s super(-1) from November 21.6 to 23.6. It was not detected after perihelion on December 3.7 when it should have been visible. We examine the active surface area necessary to explain the water production rate and its variation and are able to place constraints on the physical size of the original nucleus necessary to account for the large amount of activity from November 12.9 and until just before perihelion.
Coronal mass ejections (CMEs) play a major role in the heliosphere, and their contribution to the solar wind mass flux, already considered in the Skylab and Solwind eras with conflicting results, is ...reexamined in the light of 19 years (1996–2014) of SOHO observations with the Large Angle and Spectroscopic Coronagraph (LASCO‐C2) for the CMEs and extended for the first time to all latitudes thanks to the whole‐heliosphere data from the Solar Wind ANisotropies (SWAN) instrument supplemented by in situ data aggregated in the OMNI database. First, several mass estimates reported in the ARTEMIS (Automated Recognition of Transient Events and Marseille Inventory from Synoptic maps) catalog of LASCO CMEs are compared with determinations based on the combined observations with the twin STEREO/Sun Earth Connection Coronal and Heliospheric Investigation coronagraphs in order to ascertain their validity. A simple geometric model of the CMEs is introduced to generate Carrington maps of their mass flux and then to produce annualized synoptic maps. The Lyman α SWAN data are inverted to similarly produce synoptic maps to be compared with those of the CME flux. The ratio of the annualized CME to solar wind mass flux is found to closely track the solar cycle over the heliosphere. In the near‐ecliptic region and at latitudes up to ∼55°, this ratio was negligibly small during the solar minima of cycles 22/23 and 23/24 and rose to 6% and 5%, respectively, at the maximum of solar cycles 23 and 24. These maximum ratios increased at higher latitudes, but this result is likely biased by the inherent limitation of determining the true latitude of CMEs.
Key Points
Assess the contribution of coronal mass ejections to solar wind mass flux usingSOHO/LASCO, SOHO/SWAN, and OMNI data
Their ratio closely tracked the solarcycle over the heliosphere
It varied from negligible during the solar minima of cycles 22/23 and 23/24 to 6 and 5% at the maximum of cycles 23 and 24, respectively
Mars is believed to have lost much of its surface water 3.5 billion years ago, but the amounts that escaped into space and remain frozen in the crust today are not well known. Hydrogen atoms in the ...extended martian atmosphere, some of which escape the planet's gravity, can be imaged through scattered solar UV radiation. Hubble Space Telescope (HST) images of the ultraviolet H Ly α emission now indicate that the coronal H density steadily decreased by a factor of roughly 40% over 4 weeks, a far greater variation than had been expected. The leading candidate cause is a decrease in the source rate of water molecules from the lower atmosphere, consistent with seasonal changes and a recent global dust storm. This implies that the rate of escape of martian hydrogen (and thereby water) into space is strongly dependent on the lower atmospheric water content and distribution.
Key Points
A strong variation in escape of hydrogen from the martian atmosphere is reportedThe atmospheric escape from Mars has a strong seasonal dependenceAtmospheric escape from Mars is strongly enhanced during a global dust storm
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